(i) Field of the Invention
The present invention relates to a control system for a four-wheel drive transmission mounted on a vehicle.
(ii) Description of the Prior Art
A four-wheel drive automobile is equipped with a combination of an automotive automatic transmission including a multiratio gear unit; friction coupling devices which are operated by a hydraulic servomechanism so that the components thereof are connected to an input shaft, an output shaft or other component or selectively fixed to a fixed member; and a hydraulic control system which controls the connection of the working fluid to the hydraulic servomechanism of the friction coupling device. The automobile is further equipped with a four-wheel drive transfer unit capable of establishing either a two-wheel drive mode or a four-wheel drive mode in any of a plurality of change speed ratios (reduction ratios). Automatic change between the two-wheel drive mode, the four-wheel drive mode of a specified reduction ratio and multiratio four-wheel drive mode is achieved by selectively engaging the frictional coupling devices, such as clutches and brakes, by the hydraulic servomechanism. In a conventional four-wheel drive vehicle, however, the automatic speed change between a plurality of reduction ratios in the four-wheel drive running mode is controlled by the governor pressure which is regulated by a governor valve as an input pressure relating to the running speed, and hence the set speeds for speed change can not be optionally changed and any variation of the governor pressure causes variation of the set speeds.